As one abnormal combustion condition among various combustion conditions of an engine, air-fuel mixture is ignited by itself for some reason without ignition spark in the midst of compression stroke of the engine. This is referred to as a pre-ignition. The pre-ignition causes malfunction in engine output power or engine rotation.
It is thus necessary to control the engine by detecting such a pre-ignition. The pre-ignition may be detected generally by comparing an ignition timing and an abnormal vibration generation timing. For this detection, the abnormal vibration generation timing need be detected accurately. The pre-ignition varies its magnitude very much and frequently occurs at the same time as an engine knock. To differentiate the pre-ignition and the knock, it is necessary to detect accurately both of the magnitude and the generation timing of the pre-ignition.
JP H08-319931A, which corresponds to U.S. Pat. No. 5,632,247, discloses one example of detection of a pre-ignition by comparison of an ignition timing and an abnormal vibration generation timing. Specifically, a microcomputer (signal processing device) generates a sensor value acquisition request at every predetermined time interval and an analog-digital converter (AD converter) performs AD conversion in response to the sensor value acquisition request. The microcomputer checks whether an AD-converted sensor value exceeds a threshold value thereby to check whether an abnormal vibration is generated. The microcomputer checks whether a pre-ignition is present by comparison of the generation timing of the abnormal vibration and the ignition timing.
According to this detection method, the AD converter performs the AD conversion period in response to the request from the microcomputer. The microcomputer determines the pre-ignition when the AD-converted sensor value is higher than the threshold value and the abnormal vibration generation timing and the ignition timing are within a fixed crankshaft rotation angular interval. For accurately detecting the sensor value of the pre-ignition vibration frequency in a range of 5 kHz to 25 kHz, a sampling frequency of about 100 kHz is needed. Thus, the above-described processing need be finished within a interval of 10 microseconds (μs) and hence the microcomputer must be capable of high speed data processing.
For reducing the processing load or shortening the processing time, a pre-ignition detection interval and a knock detection interval are differentiated. However this detection method makes it impossible to detect the pre-ignition, when the pre-ignition is generated near the knock generation timing.